German patent specification No 29 38 408 discloses a guide rail for a hewing machine which has a running rail profile member comprising a holding bar portion at the filling side and a guide bar portion at the coal face side. The holding bar portion at the filling side is screwed fast to the mounting plate of the conveyor, on the filling side. Disposed at a spacing from the holding bar portion is the guide bar portion on the coal face side, while a chain passage for the chain is formed between the holding bar portion and the guide bar portion and an engagement groove for the chain wheel of the hewing machine is provided above the chain between the holding bar portion and the guide bar portion. Provided beneath the chain in the running rail profile member are discharge openings for the discharge of material to be conveyed, which is disposed in the chain passage. The guide bar portion is embraced by an L-shaped angular guide member on the hewing machine such that the contact surfaces of the guide bar portion for guiding the hewing machine is comparatively small. Consequently, the guide force bears on a comparatively small area of contact on the guide bar portion. This causes a correspondingly high rate of wear of the guide bar portion. When the guide bar portion is worn, the entire running rail profile member has to be replaced. On the other hand, the chain in the running rail profile member permits a uniform chain pitch, even in the case of angled channel runs, so that a comparatively large channel angle can be permitted without difficulties arising in reqard to movement of the hewing machine along the conveyor.
German laid open application (DE-OS) No 27 21 867 describes a drive shaft bar arrangement for a hewing machine. The drive shaft bar arrangement comprises longitudinal portions which correspond to half the length of a channel run. The junctions between adjacent channel runs are bridged over by longitudinal portions which are arranged at both ends in a respective bar holder with longitudinal play. Fixedly arranged between each two adjacent longitudinal portions with longitudinal play on the conveyor channel is another longitudinal portion. The longitudinal play of the longitudinal portions which bridge over the junctions in the conveyor channel is required in order to permit the individual channel runs to be in an angled position relative to each other. This arrangement provides comparatively large contact surface areas for a guide shoe on the hewing machine, thereby giving a good guiding effect for the hewing machine and a comparatively low rate of wear in respect of the drive shaft bar arrangement. On the other hand, however, an angled arrangement of the channel also involves a change in the pitch of the drive shaft teeth. This means that only a limited angled configuration of the channel can be permitted so that the hewing machine can move along the drive shaft bar without involving problems.
The known guide rails therefore give only the advantage of a uniform chain pitch, in which case it is necessary to accept the disadvantage of a high rate of wear and the resulting correspondingly short service times, or, only the advantage of a low rate of wear and the long service times involved, while not being able to prevent disadvantageous changes in pitch of the drive shaft tooth arrangement.